Wavelength selective optical coupling devices according to related art for optical add/drop multiplexing are used to extract a single wavelength or several wavelengths from a broadband optical signal, or to add a single wavelength to a broadband optical signal. Such devices are typically used in fiber optic networks utilising wavelength-division multiplexing, to add or drop wavelength channels to/from an optical fiber.
Most optical add/drop multiplexers used today are fixed. That is, the wavelength to be added or dropped is determined by device design and cannot be changed after the device has been manufactured. To allow for more dynamic and flexible planning of the traffic in the optical networks, reconfigurable optical add/drop multiplexers are being developed. With these devices it is possible to change the wavelength channel to be added or dropped also after the device has been installed in the network. Thus, the network can more efficiently be controlled in terms of traffic provisioning and protection switching.
A common implementation of reconfigurable optical add/drop multiplexers is to use a demultiplexer to split up the incoming broadband light signal into separate waveguides for each wavelength channel. With 2-by-2 crossbar switches on each channel waveguide, it is possible to select which wavelengths to be added/dropped. After the switches, a multiplexer is used to recombine all wavelengths channels into a common output waveguide. This kind of reconfigurable optical add/drop multiplexer is described in the review article “The wavelength add/drop multiplexer for lightwave communication networks”Bell Labs Technical Journal, January-March 1999, p. 207-209 by C.R. Giles and M. Spector.
A drawback associated with the solution above is that the multiplexers are expensive and have large transmission loss. Thus, an alternative solution, without the (de-)multiplexers, is the broadcast-and-select solution. This implementation is discussed in the paper “Broadcast-and-select OADM enables low-cost transparency”, Light-wave, December 2001 by J. Bayne and M. Sharma. In this implementation a coupler is used to tap off a part of the optical power from all channels. The tapped broadband signal is directed towards one or more narrow band channel filters. These channel filters select the wavelengths that shall be dropped from the network. The optical signal that was not tapped off by the coupler is directed towards a wavelength blocker that attenuates the wavelengths channels that were dropped by the channel filters. Thus, after the wavelength blocker, new information can be added to the dropped wavelength channels.
Some common disadvantages with both the solutions above are that many different components are required to implement the add/drop functionality. Thus, the optical add/drop device becomes expensive. Furthermore, with all the different components the light must pass, there will be substantial optical loss. This loss must be compensated for by optical amplifiers, which further increase the cost for the implementation.
Simpler optical add/drop solutions can be implemented using fiber Bragg gratings and circulators. Here the incoming broadband signal is coupled from port 1 of the circulator to its port 2, where a fiber Bragg grating is reflecting one or more wavelength channels. The reflected channels are now entering port 2 of the circulator and are dropped in its port 3. To add new information to the dropped channels, a second circulator is used. Signals that are to be added enter a second circulator and exit from there towards the fiber Bragg grating, where the selected wavelength channels to be added are reflected. The added wavelength channels then pass through the second circulator. The wavelength channels that are not reflected by the fiber Bragg grating, pass straight through the first circulator, the fiber Bragg grating and the second circulator. The implementation of this kind of optical add/drop multiplexer is also discussed in the previously mentioned review article by Giles and Spector, to which reference is made.
By means of the solution above, some complexity and thereby cost can be reduced. However, the circulators incur loss on the express traffic, i.e. the traffic that is not added or dropped, and thus only a few optical add/drop multiplexers can be added to a fiber network before optical amplifiers have to be added.
A solution directed towards coupling out a selected wavelength channel from a wavelength-division multiplexed broadband signal is disclosed in U.S. Pat. No. 4,466,694. Wavelengths that are not to be coupled out from a waveguide are meant to pass a described device with negligible loss. A disadvantage associated with the suggested implementation, however, is that light is only coupled out from the waveguide to radiation modes. Thus, the device will function merely as a wavelength-selective attenuator, but not for optical add/drop multiplexing, where light should be coupled between waveguides.
The ideas described above are further discussed in U.S. Pat. No. 6,501,879, in which two different implementations are described of how a wavelength-selective coupler according to that disclosed in U.S. Pat. No. 4,466,694 can be used for optical add/drop multiplexing. In a first example, the wavelength-selective coupler is used together with circulators, in a similar manner as the fiber Bragg grating is used in the example above. A disadvantage of this solution is that the wavelength selective coupler only reflects light, when light is coupled out orthogonally from the waveguide in to the surrounding external resonator. Thus, to be able to tune this device, the grating period in the waveguide has to be changed simultaneously with the surrounding optical resonator.
In the second optical add/drop multiplexer implementation according to U.S. Pat. No. 6,501,879, two wavelength-selective couplers are used in series to retro-reflect the wavelength channel to be dropped. However, the same disadvantage remains in this implementation as in the previously mentioned implementation, namely that the waveguide grating has to be tuned to couple the selected light orthogonally out from the waveguide and in to the external resonator.
Based on the above, a solution to the mentioned problems in association with related art is desirable.